Discrete rotary switches are a mature technology, and are still the most reliable way to control multi-circuit functions in various voltage and current applications. Such switches are actuated through the rotational action of a shaft. Depending upon the design, the shaft may rotate through a 360 degree range or any portion thereof. Further, the shaft may rotate in either direction (clockwise or counter clockwise) and may be capable of continuous rotation in a chosen direction.
Rotary switches may be of single plane or "deck" construction or may be comprised of a stack of decks which may or may not be similar. Rotary switches come in many sizes, ranging from relatively small for installation on printed circuit boards, to relatively large for panel mounting.
Depending upon the specific application, the rotary switch can be subjected to all types of ambient conditions. Accordingly, rotary switches are constructed in many different ways. Some are designed to merely perform the multi-circuit function with little or no concern for the ambient environment, while others are designed and constructed to protect their operation from many different, often hostile environmental conditions, thereby assuring high reliability and dependability and an extended useful life.
While rotary switches provide a unique multi-circuit function feature, they also provide another major advantage. They are less prone to accidental actuation and can include further safety features such as "pull to turn" or "push to turn" operating modes, as well as latches and locks.
The modern rotary switch is found in commercial, industrial and governmental applications. Military specifications, such as MIL-S-3786, have been promulgated for rotary switches. These impose upon manufacturers certain requirements of standardization and performance for switches that are qualified under such specifications.
Rotary switches of the prior art have generally been made up of modular annular frame members, including frame members which hold a common conducter that is contacted at all times and frame members holding a plurality of conductive segments (the conductive surface areas) which are contacted in a predetermined sequence as a movable contact rotates. The movable contact closes a circuit between the common conductor and the individual conductive segments. These conductive segments incorporate a termination extension of themselves to the exterior of the switch housing, which extensions are terminals to which conductors can be physically and electrically attached.
When the switch is assembled, the frame members are held together by elongated bolts which can compress the frame members into a single assembly. Depending upon the many circuit control configurations available, this type of switch can be utilized to control from one to more than one hundred different circuits.
While acknowledging all of the positive features and advantages of the rotary switch, attention must also be given to those areas of a less positive nature. These include the manner of interconnecting the switch into an application and the ever increasing need to control low level TTL logic signals and so called "dry circuits", which utilize voltage and current signals at extremely low levels.
The normal and customary installation of a switch into a system requires that conductive wires or leads be affixed to the various terminals on the exterior surface of the switch by a soldering process. Unfortunately, the soldering procedures, especially those conforming to government specifications such as WS6536 (a Navy weapons specification) induce a very high temperature at the terminals to initiate the flow of soldering flux over the surface of the affected terminal. Due to the high heat levels, the flux follows the terminal surface and often penetrates the housing, reaching the contact surface on the interior of the switch. The flux, being a non-conductive contaminant, may result in an "open circuit" at best, and may chemically attack the operating components, at worst.
In addition to the concerns of flux contamination, another problem area in prior art rotary switches resides in the mechanism utilized to transport the common conductor to the selected contact point. The nearly universal prior art method employed for moving the conductor from contact point to contact point has been a sliding or "wiping" motion which causes the contact to traverse the plastic material in which the contact points are embedded.
The stated goal of the sliding or wiping contact is to keep the contact points "clean" and free of contaminants such as metal oxides and the like. However, as the conductor slides on the plastic material, it can pick up particles of plastic which are then transferred to the individual contact points and which, in time, become spread over the surface, increasing the contact resistance.
In extreme cases, this contamination can result in an open circuit or in an intermittent fault. Further, the increased resistance can cause arcing which can melt or char the plastic, increasing the contamination. The problem of plastic contamination is especially serious when dealing with low level TTL and/or "dry circuit" applications in which less than five milliohms resistance is a requirement.